Production of monocalcium orthophosphate

ABSTRACT

A multicycle process for the continuous production of highpurity monocalcium orthophosphate which may, if desired, be used as a fertilizer material. The monocalcium orthophosphate is produced by the digestion of phosphate raw material with phosphoric acid to form a solution containing monocalcium orthophosphate which is then crystallized and recovered from solution. A portion of the recovered salt is removed as the end product. The remaining portion is contacted with a cationic exchange resin on the hydrogen cycle to produce phosphoric acid which is recycled for use in the digestion of more phosphate raw material.

United States Patent Cochran 51 *Feb. 29, 1972 [5 1 PRODUCTION OFMONOCALCIUM 2,567,227 9/1951 Miller ..23/109 ORTHOPHOSPHATE 3,374,0553/1968 Vi11alon.... ....23/l07 3,494,735 2/1970 Cochran... ..23/165 [72]inventor: Linden Wayne Cochran, Basking Ridge,

Primary Examiner-Earl C. Thomas Assistant Examiner-Gregory A. Heller[73] Assignee. Multl Mlnerals Limited, Toronto, Ontario, Anomey NormanM- Holland Canada Notice: The portion of the term of this patent sub-ABSTRACT seqPem 1987 has been A multicycle process for the continuousproduction of highclalmed' purity monocalcium orthophosphate which may,if desired, be 22 Filed; Man 4 9 used as a fertilizer material. Themonocalcium orthophosphate is produced by the digestion of phosphate rawPP 4, material with phosphoric acid to form a solution containingmonocalcium orthophosphate which is then crystallized and recovered fromsolution. A portion of the recovered salt is [52] U.S.Cl ..23/109,23/165 removed as the end producL The remaining Portion is Com [51] Int.Cl. ..C0lb 25/22, C01b 25/32 meted with a cationic exchange resin on thehydrogen cycle to [58] Fleltl of Search ..23/ 108, 109, 165C producephosphoric acid which is recycled f use in the 6 f C digestion of morephosphate raw material. [5 Re erences ited 4 Claims N0 Drawings UNITEDSTATES PATENTS PRODUCTION OF MONOCALCIUM ORTHOPHOSPHATE CROSS REFERENCEThe present application is an improvement over my US. Pat. applicationSer. No. 386,764 filed July 3l, 1964, now US. Pat. No. 3,494,735.

DESCRIPTION The present process is directed to a method for thecontinuous production of high-purity monocalcium orthophosphate, whichis commonly referred to as triple super phosphate, and which is suitablefor use in high-analysis fertilizers.

It has been a common practice to produce phosphate salts suitable foruse in fertilizers from the so-called wet process phosphoric acids,namely those produced by the acidic digestion of phosphate raw materialwith strong mineral acids. An example of such a process is shown 'inU.S. Pat. No. 3,044,851. Unfortunately, such a product is more impurethan desired in high-grade fertilizers since it contains all theadditional anionic impurities derived from the initial reaction of thestrong mineral acid and phosphate raw material.

ln view of this shortcoming, a number of processes which utilizephosphoric acid as the digesting acid to produce monocalciumorthophosphate have been developed. Examples are found in US. Pat. Nos.2,899,292; 2,914,380; 2,567,227 and 3,374,055. However, in none of theseprocesses was it appreciated that the reaction conditions should producea solution of monocalcium orthophosphate which can be easily removedfrom the mother liquor with a portion used as the end product while theremaining portion of monocalcium salt is contacted with an ion exchangeresin to produce phosphoric acid which can be recycled to subsequentdigesting steps.

It is an object of this invention to provide a process which is suitablefor production of high-quality monocalcium orthophosphate fertilizermaterial.

It is another object of this invention to provide a multicycle processfor the production of monocalcium orthophosphate which is continuouswith respect to the production of that salt.

These and other objects not explicitly set forth in this specificationor the appended claims will become obvious to one skilled in the artupon examination of theembodiment about to be described or upon use ofthe invention in practice.

The present process is directed to production of monocalciumorthophosphate by the acidic digestion of phosphate raw material, suchas apatite, with phosphoric acid.

The reaction conditions should be such that a solution containingrecoverable monocalcium orthophosphate is produced. Preferably, thedigesting acid should be between about 55 percent and 85 percent H POand the digestion temperature is maintained from about 85 C. to about105 C. A suitable process is described in Belgian Pat. No. 657,470. Inorder to aid in the removal of fluorine contaminants contained in theraw material air-sparging currents can be introduced into the digestionmedium. Other steps may also be used to further remove fluorinecontaminants either in lieu of or in combination with the spargingtechnique. For example, addition of silica to the digestion medium, andthe application of a slight negative pressure to the digester furtheraid in the removal of volatile contaminants.

The monocalcium orthophosphate product contained in the solution is thencrystallized from solution by allowing the solution to cool to between70 C. and 85 C. The crystals may then be recovered by conventionalfiltration or centrifugation and decantation procedures. They may bewashed with either an aqueous or phosphoric acid solution saturated withmonocalcium orthophosphate or concentrated phosphoric acid to removeentrained contaminants.

Once the monocalcium orthophosphate salt has been recovered a portion isused as the desired end product of this process. The remaining portionis contacted with a cationic exchange resin on the hydrogen cycle toconvert the salt to phosphoric acid. The salt may be contacted while inthe crystalline state of after solubilization in a suitable solvent suchas described in Belgian Pat. No. 657,470. This highly pure acid isrecycled for use in the digestion step. The result is a multicycleprocess which continuously produces monocalcium orthophosphate while atthe same time producing a suffrcient amount of acid for use in latercycles. The process is illustrated by the following example:

EXAMPLE Approximately 50 liters of mother liquor was prepared byreacting percent H PO with Kola apatite at a temperature of about C. Thereaction vessel was agitated with a mechanical stirrer throughout thereaction interval and the solution was air sparged to assist in removalof liberated fluorine. The solution was continuously cycled between two30-liter containers, passing through a filter device en route from thereaction vessel to the holding vessel. Apatite was fed to the reactor ata constant rate of about 100 grams per minute, and the solution wascycled at a rate of 1 liter per minute..

As soon as visible crystals of monocalcium orthophosphate appeared inthe reactor the filtering unit was activated. The crystallinemonocalcium orthophosphate was filtered from the natant mother liquor,washed with 65 percent weight percent H PO weighed and reserved forfurther treatment as indicated below.

Approximately 2,325 grams of monocalcium orthophosphate was filteredfrom 3.88 liters of mother liquor. The filtered crystals were washedwith 1,715 milliliters of 65 percent weight percent H PO which hadpreviously been saturated with the monocalcium salt to prevent erosionof the filter cake by the wash acid. The amount of acid used to wash thecrystals was the stoichiometric quantity required to produce that amountof monocalcium orthophosphate from apatite at the rates given above.Included in the wash acid was a quantity of acid sufficient to offsetthat normally lost in the cake through drag-out. This quantity wasapproximately 20 percent in the system above.

Approximately 1,350 grams or about 60 percent of the product, containing5.775 equivalents of Ca *ion was contacted with 3.21 liters of strongacid ion exchange resin in the hydrogen form (Dowex 50W-X8) to absorbthe calcium ion and to convert the salt to phosphoric acid according tothe reaction:

Ca(l-l PO +RH =2H PO +RCa A total of 1,820 grams of phosphoric acidproduct were obtained, with a specific gravity of 1.252. This acid,approximately 40 percent weight percent concentration, was thenevaporated to 65 percent weight percent for further utilization, firstas wash acid, then to react in the reaction vessel with apatite for thefurther production of monocalcium phosphate.

The remaining 975 grams of monocalcium orthophosphate, or about 40percent of the salt, representing net product from converting apatite tothe monocalcium form, was then available for use either as directapplication fertilizer (triple super phosphate, without the usualimpurities), for mixing with nitrogenous and potash materials for use asmixed (complete) plant foods, or for further treatment. The monocalciumsalt contained less than 50 ppm. fluorine similar amounts of iron and nosulfate ion.

As many and varied modifications of the subject matter of this inventionwill become apparent to those skilled in the art from the detaileddescription given herein, it should be understood that this invention isto be limited only in accordance with the appended claims.

I claim: orthophosphate 1. In the process for the manufacture ofphosphoric acid which comprises digesting a phosphate containingmaterial with phosphoric acid to form a mother liquor containingmonocalcium orthophosphate, crystallizing said monocalciumorthophosphate, removing said monocalcium orthophosphate crystals fromsaid mother liquor and contacting said monocalcium orthophosphatecrystals with a cationic ion exchange carried out at about C. to aboutC. using a phosphoric acid of from about 55 percent to 85 percent.

3. A process according to claim 2, in which the phosphatecontainingmaterial is apatite.

4. A process as claimed in claim I, in which the monocalciumorthophosphate is solubilized and is contacted with the resin while itis solubilized.

2. A process as claimed in claim 1, in which the digestion is carriedout at about 85* C. to about 105* C. using a phosphoric acid of fromabout 55 percent to 85 percent.
 3. A process according to claim 2, inwhich the phosphate-containing material is apatite.
 4. A process asclaimed in claim 1, in which the monocalcium orthophosphate issolubilized and is contacted with the resin while it is solubilized.